1. Field of Invention
The present invention relates to a code division multiple access (CDMA) system for network communication. More particularly, the present invention relates to a CDMA system whose data reception and data transmission are carried out using different channel cards.
2. Description of Related Art
Due to rapid advancements in communication technologies, the number of mobile telephone users is increasing at a lighting pace. To improve the quality of communication, better communication techniques (such as wide band networks) are developed along with the construction of more base stations.
FIG. 1 is a block diagram showing a conventional CDMA system. As shown in FIG. 1, an antenna 10 is used to receive radio frequency signals emitted by a particular user or to broadcast radio frequency signals to users. A radio frequency module 12 is a device that converts any received middle frequency signals into radio frequency signals and then transmits the radio signal from the antenna 10. Similarly, the radio frequency module also converts the radio frequency signals picked up by the antenna 10 into middle frequency signals.
A middle frequency module 14 is an interface that takes care of input/output functions between the radio frequency module 12 and a plurality of channel cards 16. Each channel card 16 comprises a digital/analogue converter 18, an application specific integrated circuit/field programmable gate array 20 and a processor 22. The channel card 16 processes base-band digital signals. The digital/analogue converter 18 inside the channel card 16 is responsible for converting base-band digital signals to analogue middle-frequency signals and then transmitting to the middle frequency module 14. Similarly, the digital/analogue converter 18 is also responsible for converting analogue signals from the middle frequency module 14 into base-band digital signals to be used by the channel card 16.
The application specific integrated circuit/field programmable gate array 20 serves many functions inside the channel card 16 including power control, code spreading/de-spreading, modulation/demodulation, combining modules, Gold/Walsh code generation, RAKE receiver, peak detection, tracking loop, automatic frequency control, channel estimation and receiver synchronization.
Besides a power control function, the processor 22 inside the channel card 16 performs cyclic redundancy check, cyclic redundancy check decoding, convolution encoding, rate matching, rate matching decoding, interleaving encoding, bus connection between channel card and channel master, Viterbi decoding and interleave decoding.
A back-haul interface module 24 takes care of the input/output transmission between the channel cards 16 and a controller 26 of a cable-network control station. The controller 26 functions as an electronic signal switcher.
In a CDMA communication station design, base-band digital signal processing is a critical technique. However, a number of technical problems are often encountered in the design of a CDMA communication station, including: (1) each channel card has to process signal reception and signal transmission independently; however, due to the concentration of output terminals (such as middle frequency module, radio frequency module and antenna), various channel cards have to exchange data with each other between reception and transmission; hence, complicated channel combining computations (energy-contained control) must be processed by the same processor/application specific integrated circuit leading to overloading and complexity of transmission signals in each channel card; (2) each channel card must be able to receive all the information from the other channel cards for transmission integrity; hence, extra hardware must be available to deal with coding/decoding and error correction leading to design complication for each channel card; (3) When various channel cards transmit middle frequency signals to the middle frequency module, due to the simple interference of the buses from the various channel cards to the middle frequency module and the concentration of various channel card components, transmission errors are very difficult to rectify; hence, system reliability is low.